mikedilger on Nostr: You can't in general derive two different NPUBs from the same NSEC, or have two ...

You can't in general derive two different NPUBs from the same NSEC, or have two different NSECs for the same NPUB. There might be high order reflections/overrlaps, but they end up being mathematically equivalent. For example, these two NSECs are the same:

3501454135014541350145413501453fefb02227e449e57cf4d3a3ce05378683
cafebabecafebabecafebabecafebabecafebabecafebabecafebabecafebabe

But most nsecs won't have a reflection.

IMHO if nostr is to move to a masterkey-subkey situation, we should use that opportunity to allow for different kinds of keys and different cryptosystems. I want an ed25519 device key issued by my master nsec (and if nostr doesn't support it I don't care because I can still use it in my own projects). Ideally I'd want an ed25519 master key but I predict that nostr won't move in that direction because of the "one way" rule.

Also, I want my current keypair to be a device key under a master key that doesn't even exist yet. Because everybody already knows me by this keypair. Clearly it cannot be derived from the future master key.

For both of these reasons, I don't think deriving device keys from a master key is going to work. I think they should be independent and simply one signs the other.